406 error; Route error after executing some JavaScript

I have a website running on a Linux server: www.ultradedup.com

I can go to www.ultradedup.com on one computer or another browser and
everything works fine.

Processing HomeController#index (for at 2011-05-30
22:05:38) [GET]
Parameters: {“action”=>“index”, “controller”=>“home”}
Rendering template within layouts/application
Rendering home/index
Completed in 6ms (View: 6, DB: 0) | 200 OK [http://www.ultradedup.com/]

When I go to www.ultradedup.com/usage/BuildBatches.html everything
appears to work. I render a fair amount of Javascript code which appears
to work.

When I then attempt to access www.ultradedup.com I get

Processing ApplicationController#index (for at 2011-05-30
22:11:40) [GET]

ActionController::RoutingError (No route matches “/406.shtml” with
haml (3.0.24) [v] rails/./lib/sass/plugin/rack.rb:41:in call' config/initializers/mongrel.rb:63:in dispatch_cgi’

Rendering /home/ultraded/rails_apps/UltraDedupRoR/public/404.html (404
Not Found)

I am lost as to how some (what I hope is innocuous JavaScript code) can
cause this problem.

Restarting the server does not clear the problem.

Clearing the session cookie does not clear the problem.

The problem is not exhibited when I use Webrick.

The server has not died. Other computers can access the website as long
as they don’t attempt to execute the JavaScript code. Running other
browsers on the same computer works as long as I don’t run that
JavaScript code.

Any ideas as to what is causing this? I read some stuff about how rare
406 errors are but this has caused IE9, Firefox, and Opera to report the
406 errors.